Solid state acoustic wave devices, including surface acoustic wave (SAW) and shallow bulk acoustic wave (SBAW) devices, utilize metal transducers on the surface of piezoelectric crystals to interact with the various modes of wave propagation in the crystal. The relatively thin transducers are subject to chemical reactions with various atmospheric or other contaminants which may alter or destroy their usefulness.
It is known in the art that solid state acoustic devices may be hermetically sealed to prevent such corrosive degradation. In addition to being relatively costly, such a hermetically sealed container may contain matching circuits which out-gas harmful contaminants or may otherwise seal in contaminants.
Other known passivation techniques, such as the application of a thin layer of Si.sub.3 N.sub.4 or glass are not generally useful for SAW and SBAW devices because the coating on the surface of the crystal produces undesirable loading of the surface. In addition, such passivation processes must be carried out prior to packaging the device and pathways must be opened in the passivation layer for bonding leads. These pathways provide an opening for etchants.
Low scatter SAW devices are known in the art for providing efficient coupling of the transducers with surface modes of the crystal while suppressing scattering of the surface waves into bulk modes of the crystal. In the prior art, low scatter transducers are formed by depositing an insulating material between the previously formed fingers of the inter-digital transducer. This method leaves gaps between the insulating filler and the electrode fingers, thus reducing effectiveness.